My previous pass missed these atomic operations using operators.

This only adds the rbimpl_ version to include/ruby/atomic.h so that it
is not a new public interface.

We were already using RUBY_ATOMIC_VALUE_LOAD in a few locations. This
will allow us to use other memory orders internally when desired.

"store" is the terminology the C11 standard uses, which allows us to use
this as a fallback.

This only changes the private rbimpl_ version of the method,
RUBY_ATOMIC_SET et al. keep the same name.

The implementation is only active if `HAVE_STDATOMIC_H` is defined,
and only after the compiler fails to match all currently supported
systems.

These branches are only active for 32-bit Windows and Solaris platforms,
codify the fact by changing `#else` to `#elif`'s that explicitly include
those targets and `#error`-out otherwise.

If the object isn't shareable and already has a object_id
we can access it without a lock.

If we need to generate an ID, we may need to lock to find
the child shape.

We also generate the next `object_id` using atomics.

If the shape has only one child, we check it lock-free without
compromising thread safety.

I haven't computed hard data as to how often that it the case,
but we can assume that it's not too rare for shapes to have
a single child that is often requested, typically when freezing
and object.